Power transmissions are used to change the rotational speed of drivers to a selected speed suitable for the driven load. There are many types of transmissions. Two main categories in power transmissions are gear type and belt type transmissions. Gear type power transmissions include, for example, parallel shaft gear boxes, worm gear reductions and planetary gear drives. Belt type power transmissions include flat belt, V-belt, V-ribbed belt, and timing belt, and cable chain transmissions.
When transmission devices employ gears to provide an increased or reduced rate of rotation of an output shaft in relation to the rate of rotation of an input shaft, the gears ensure that slippage does not occur between the input and output shafts. Such slippage may occur in certain belt type transmissions.
Gear transmissions have the advantage of high horsepower ratings, high efficiency (with exception of the wonn gear reductions) and relatively small size. Even though gear transmissions have certain advantages, gear transmissions also have limitations. Among the limitations associated with gear transmissions, are that the teeth of the gears and the bearing and shaft configuration must be machined accurately in order to minimize frictional resistance and to provide an accurate angular relationship between the input and output shafts. As a result, gear transmissions are relatively expensive when compared to an equivalent belt type transmission.
Another type of gear transmission is called a planetary gear transmission or drive. A planetary gear system includes a sun gear, multiple planet gears and a ring gear. The sun gear includes gear teeth on its outer surface. The ring gear includes gear teeth on its inner diameter. Usually three or more planet gears mesh with the gear teeth on the sun and on the inner diameter of the ring gear. The planet gears are positioned between the sun gear and the ring gear. In operation, the planet gears orbit around the sun when the ring gear is fixed.
The planetary gear drive has all the advantages of gear type transmissions plus the additional advantage of having the driving shaft and the driven shaft in one line (collinear input/output shafts). One limit on planetary gear systems relates to possible transmission ratios. The upper limit on the gear ratio of the planetary gear system is about 6:1. The upper limit on planetary gear systems
gear ratio is actually a physical limitation. In order to operate smoothly, the planet gears must be able to mesh with both the ring gear and the sun gear. The transmission ratio of a planetary gear transmission is equal to twice the sum of the planet radius and the sun radius divided by the sun radius. The diameter of the ring is equal to the sum of the sun diameter and twice the planet diameter. The physical limitation of fitting three or more planet gears in a ring gear without interference between the planets or mesh interference between a planet gear and ring gear limits the maximum radius of a single stage planetary gear transmission to about 6:1.
Gears operate under a diversity of conditions, and the methods of lubrication will vary accordingly. For unenclosed or exposed gearing, the lubricant is applied by an oilcan, a drip oiler, or a brush for open gears. Frequent applications of small amounts of lubricant are preferable to large volumes at longer intervals. If the gears are exposed to water or acids, a sticky lubricant that will adhere to the metal must be used.
When gears run in an enclosed casing, the larger gears may dip into a bath or oil, which will be carried to the wearing surfaces and run down into the bearings, the orientations of transmissions of this type is critical since an oil pan must be positioned for gravity feed of the lubricant. Sometimes enclosed gearing is lubricated by spraying a jet of oil on the working surfaces as they revolve toward each other. This type of lubrication requires a hydraulic pump added to the transmission design. When the contact pressure is very high, extreme-pressure EP lubricants must be used to prevent rupture of the oil film and the resulting metal-to-metal contact of the parts. Lubricants of the EP type contain additives that increase the load-carrying properties as well as prevent the squeezing out of the lubricant. Of course, foreign matter in the lubricant can cause a rapid increase in the rate of wear of the teeth whether the gear transmission is enclosed or unenclosed.
The alignment of gears is very important. Care must be exercised that shafts are parallel and within tight tolerances or the entire load will be carried at the sides of the teeth instead of across the entire width of face. Excessive wear and danger of failure result if gears are improperly mounted and aligned. The same effect occurs if the teeth are not cut parallel to the axis of rotation or may have other inaccuracies. Elastic deformation of the teeth, blanks, shafts, and bearing supports can also be the cause of misalignment. Unbalanced rotating masses in a geared system can also cause unexpected loading which the teeth are called upon to carry. Noise, vibration and shortened life can result from many causes, which may be difficult to locate and analyze.
Backlash is another limitation associated with gear transmissions. Backlash is the amount by which the width of a tooth space exceeds the thickness of the engaging tooth measured on the pitch circle. Backlash does not adversely affect proper gear function except for lost motion upon reversal of gear rotation. In a planetary gear transmission as well as other gear transmissions this lost motion is amplified by the number of gears which must be reversed. In many applications, backlash is not a problem. Other gear transmission applications will not tolerate backlash. For example, gear transmissions used in X-Y plotters or used in software controlled milling machines must be capable of moving in forward and reverse with little to no backlash so that machining or drawings can be made accurately. Forming gear transmissions with little backlash requires closer tolerances or more exacting placement and adjustment of the individual gears during assembly of the gear transmission. This adds to the cost of a gear transmission. To have zero backlash, a gear transmission requires a double gear set that is spring-loaded to take up the play. This added mechanism also adds to the cost of a gear transmission.
The major disadvantages of gear transmissions are relatively high cost, their need for accurate tolerances of the gears, shafts and bearings, their sensitivity to misalignment (axial and angular misalignment), the existence of backlash, and their need for lubrication and maintenance. An exception is small plastic gears with a relatively low power rating do not need lubrication. Splash or bath-lubricated gears are limited with respect to possible mounting angles of the transmission. Elimination or lessening the effects of backlash requires a gear transmission with a double gear set that is spring-loaded to take up the play.
Belt transmissions have the advantage of being less expensive than equivalent gear drives in that alignment of the various components is not as critical, and machining and assembly does not have to be as precise. A belt transmission is typically quieter than a gear transmission and typically has similar efficiencies. Belt transmissions require no lubrication other than possibly the bearings and this can be eliminated by using maintenance-free bearings such as sealed ball bearings or bushings. Furthermore, belt transmissions have a very smooth operation, do not require the accurate manufacturing tolerances of a gear transmission and are more forgiving in terms of alignment. The major disadvantages of belt drives are the occurrence of slip between the belt and the pulleys (this is not the case for timing or synchronous belts). Furthermore, since current one stage belt transmissions do not have the input shaft and output shaft in one line, belt transmissions generally require more space than a planetary gear transmission.
As noted above, both gear transmissions and belt transmissions have disadvantages. There is a need for a transmission that overcomes many of these disadvantages and which has most of the advantages of both the gear transmissions and the belt transmissions. There is a need for a transmission which has a high transmission ratio and which requires no lubrication and therefore no maintenance if maintenance-free bearings are used. There is also a need for an efficient transmission which is quiet, compact, and inexpensive to build and manufacture. There is also a need for a transmission that allows for mounting in any orientation and which is more forgiving in both angular alignment and axial alignment of the various components forming the transmission system. There is also a need for a transmission that is less expensive to build than a gear type transmission and, preferably a transmission that can be made with mostly off the shelf components. There is also need for a belt transmission that is capable of transmitting comparable amounts of power as a gear transmission without slippage. There is also a need for a transmission that is relatively light weight and small for the amount of horsepower that can be transmitted. There is a further need for a transmission that is smooth and table during its operation. There is further need for a transmission that has no backlash.